Retrovirus production & media color (pH)

Hi, all.
I have been trying to produce retroviruses by transfecting GP2 cells with lipofectamine.
According to a protocol, I should collect viruses between 48 to 72hr post transfection,
but the media turns yellow (no red at all) at around 40hr.
Strange thing was that the cell never reached the confluency.
Viruses collected in the condition worked very poorly.

I asked to many people, but they told me they never experienced that.
I don't know what's wrong and what to do...

Somebody with similar experience?
If you do, how did you solve the problem?

It could be the transfection - do you have an untransfected control (you should always have one of these) and a transfection reagent only control? If so, have a look at those, how do they look compared to the transfections?